ANKARA, March 6 (Xinhua) -- Relations between Turkey and Germany, which have been tense since last year, worsened further lately as the two NATO allies and key economic partners are locked in acrimony over a ban preventing Turkish ministers from addressing expats inside Germany to campaign for constitutional reforms granting extended powers to President Recep Tayyip Erdogan.

Turkish voters will go to the polls on April 16 to approve or reject constitutional amendments to transform the nation's parliamentary system into a presidential one.

Germany, home to over 3 million out of 5 million people of Turkish origin in Europe, is a key factor in the political campaign. With some 1.4 million voters, it is a must-go spot for members of the Turkish government.

Erdogan, who had campaigned in Germany in 2011 and 2014, was reportedly planning a another political rally to secure the important diaspora before the referendum.

The German government under Chancellor Angela Merkel is under increased pressure from the opposition and the press to reject such Turkish campaigns inside the country, especially after a reporter of the prominent newspaper Die Welt, Deniz Yucel, became the first German citizen to be arrested last month for what was described as "terrorist propaganda" as part of Erdogan's crackdown in the wake of the failed coup last summer.

The journalist has been accused personally by Erdogan of being a "German agent" and a member of the Kurdistan Workers' Party (PKK).

Yucel "hid in the German embassy as a member of the PKK and a German agent for one month," Erdogan said.

"We cannot understand and thus accept such an opposition to Turkish government members wanting to reunite with their citizens and speak to them from a country who declares itself as a champion of freedoms," a Turkish diplomatic source said on condition of anonymity.

"We surely don't expect such a stance from a country that we consider an ally and a friend," added the source.

Several Turkish ministers were blocked in the past few days to attend public rallies in German towns, prompting immediate and harsh criticism from Ankara.

One of them was Justice Minister Bekir Bozdag, a loyal political ally of Erdogan.

"Not allowing the Turkish justice minister to speak, does it adhere or not to German human rights, Mrs. Merkel?" Bozdag asked.

Merkel insisted that the decision was taken by the municipalities and that Germany is keen on insuring the freedom of expression.

In an attempt to defuse a very possible crisis, Merkel called his Turkish counterpart on Saturday and the latter described their conversation as "good and productive."

The Turkish prime minister told reporters that the foreign ministers of the two countries would meet this week to discuss the issue.

The growing row is troubling for Merkel because she had persuaded Turkey to help block the surge of Syrian refugees fleeing to the European Union.

But the war of words continues to escalate when Erdogan himself made harsh accusations against Germany.

On Sunday, the Turkish president accused German authorities of using "Nazi methods of the past."

"Turkey has no democracy lesson to take" from Berlin, he said.

"I, Erdogan, will decide to go to Germany and they will not allow me in the country. That's unheard of," he said. "If I want, I will go there tomorrow and let us see what they will do then."

"I thought that Nazi methods were a thing of the past in Germany," an angry Erdogan told reporters on Sunday evening. "I'm totally mistaken."

The turmoil in relations between Berlin and Ankara has been ongoing since Germany criticized the large-scale crackdown on suspected coup-plotters and those alleged to have links to Kurdish militants following last July's failed coup.

Germany also refuses to extradite dozens of Turkish individuals among their officers and diplomats that Ankara suspects of involvement in the botched coup.

"Germany does not want the political polarisation that we witness in Turkish society to spill on their land, fearing for a disruption of the public order," Huseyin Bagci, a professor from the Middle East Technical University (Metu) in Ankara, told Xinhua.

By its unwillingness to welcome Turkish government officials, Berlin is clashing with its own democratic values vocally preached in the European Union, said the expert on international relations.

"A Turkish president has the right to meet his citizens in any foreign country," added Bagci.

After Austria, the Netherlands also joined Germany in banning Turkish political rallies.

Dutch Prime Minister Mark Rutte said a planned pro-Erdogan rally in Rotterdam on March 11 would be "undesirable."

"We believe that the Dutch public space is not the place to conduct another country's political campaign," Rutte said in a statement on Friday, drawing an angry response from Turkish Foreign Minister Mevlut Cavusoglu the next day.

"What do you mean we can't campaign? Where is democracy? Where are freedoms? Where is freedom of expression?" Cavusoglu asked.

"None of you can stop us," he said. "We will go where we want to go. We will meet with our citizens and we will have our meetings."

Bozdag, the Turkish justice minister, has accused "several" EU countries of being against changing Turkey's governing system, because they do not want Turkey to be a strong and stable country.

Deniz Zeyrek, a renowned columnist from Turkish newspaper Hurriyet, while denouncing Germany, also criticized the Turkish government for using the emergency rule imposed after the failed coup to ban all opposition assemblies and meetings across the country.

"Freedom of speech goes for everyone, for Germany as well as Turkey," he said. "Let us not all forget that important fact."